None.
Mississippi Code
§ 63-7-55. Mufflers
Every motor vehicle shall at all times be equipped with a muffler in good working order and in constant operation to prevent excessive or unusual noise and annoying smoke. No person shall use a muffler cutout, bypass or similar device upon a motor vehicle on a highway.
Antique
Available for vehicles that are at least 25 years old.
Street Rod
Available for modified antique vehicles produced by an American manufacturer in 1948 or earlier that have undergone some type of modernizing, including modernization of the engine, transmission, drivetrain, interior refinements and any other modifications the builder desires. Street rods are to be driven under their own power and used as safe, nonracing vehicles for family enjoyment.
YOM
Vehicles registered as "Antique" or "Street Rod" may display YOM plates with DMV approval.